Discharge device having cathode with micro hollow array

ABSTRACT

A discharge device for operation in a gas at a prescribed pressure includes a cathode having a plurality of micro hollows therein, and an anode spaced from the cathode. Each of the micro hollows has dimensions selected to produce a micro hollow discharge at the prescribed pressure. Preferably, each of the micro hollows has a cross-sectional dimension that is on the order of the mean free path of electrons in the gas. Electrical energy is coupled to the cathode and the anode at a voltage and current for producing micro hollow discharges in each of the micro hollows in the cathode. The discharge device may include a discharge chamber for maintaining the prescribed pressure. A dielectric layer may be disposed on the cathode when the spacing between the cathode and the anode is greater than about the mean free path of electrons in the gas. Applications of the discharge device include fluorescent lamps, excimer lamps, flat fluorescent light sources, miniature gas lasers, electron sources and ion sources.

This application is a divisional of application Ser. No. 09/533,008filed Mar. 22, 2000 entitled DISCHARGE DEVICE HAVING CATHODE WITH MICROHOLLOW ARRAY, which is a division of application Ser. No. 09/310,817filed May 12, 1999 and now issued as U.S. Pat. No. 6,072,273, which is adivision of Ser. No. 08/901,195 filed Jul. 28, 1997 and now issued asU.S. Pat. No. 5,939,829, which is a division of application Ser. No.08/403,477 filed Mar. 14, 1995 and now issued as U.S. Pat. No.5,686,789.

FIELD OF THE INVENTION

This invention relates to gas discharge devices and, more particularly,to gas discharge devices which utilize a cathode having a micro hollowarray.

BACKGROUND OF THE INVENTION

The general concept of a discharge device which utilizes a hollowcathode for increased current capability is disclosed in the prior art.A hollow cathode glow discharge utilizing a single, nearly sphericalhollow cathode is described by A. D. White in Journal of AppliedPhysics, Vol. 30, No. 1, May 1959, pp. 711-719. The author reported astable discharge and negligible deterioration from sputtering. The basicmechanisms contributing to the hollow cathode effect are described by G.Schaefer et al. in Physics and Applications of Pseudosparks, Ed. by M.A. Gundersen and G. Schaefer, Plenum Press, New York, 1990, pp. 55-76.Measurements of the temporal development of hollow cathode dischargesare described by M. T. Ngo et al. in IEEE Transactions-on PlasmaScience, Vol. 18, No. 3, Jun. 1990, pp. 669-676.

A variety of hollow cathode structures for fluorescent lamps have beendisclosed-in the prior art. A directly-heated hollow cathode having aninterior coating of an emissive material is disclosed in U.S. Pat. No.4,523,125, issued Jun. 11, 1985 to Anderson. A shielded hollow cathodefor fluorescent lamps is disclosed in U.S. Pat. No. 4,461,970, issuedJul. 24, 1984 to Anderson. A hollow electrode having an interior coatingof an emissive material that varies in thickness is disclosed in U.S.Pat. No. 2,847,605, issued Aug. 12, 1958 to Byer. A short arcfluorescent lamp having hollow cathode assemblies is disclosed in U.S.Pat. No. 4,093,893, issued Jun. 6, 1978 to Anderson. Cup shapedelectrodes containing emissive material for use in cold cathodefluorescent lamps are disclosed in U.S. Pat. No. 3,906,271, issued Sep.16, 1975 to Aptt, Jr., and U.S. Pat. No. 3,969,279, issued Jul. 13, 1976to Kern. A fluorescent lamp wherein a filament is positioned within acylindrical shield is disclosed in U.S. Pat. No. 2,549,355, issued Apr.17, 1951 to Winninghoff. Additional hollow cathode discharge devices aredisclosed in U.S. Pat. No. 1,842,215, issued Jan. 19, 1932 to Thomas;U.S. Pat. No. 3,515,932, issued Jun. 2, 1970 to King; U.S. Pat. No.4,795,942, issued Jan. 3, 1989 to Yamasaki; U.S. Pat. No. 3,390,297,issued Jun. 25, 1968 to Vollmer; and U.S. Pat. No. 3,383,541, issued May14, 1968 to Ferreira.

An electrical discharge electrode including a plurality of tubes, whichare filled with an electron emissive material and embedded in a metallicmatrix, is disclosed in U.S. Pat. No. 4,553,063, issued Nov. 12, 1985 toGeibig et al.

A variety of.different fluorescent lamp types have been developed tomeet different market demands. In addition to conventional tubularfluorescent lamps for office and commercial applications, compactfluorescent lamps have been developed as incandescent lamp replacements.Subminiature fluorescent lamps have found applications in displays andgeneral illumination in limited spaces.

Different fluorescent lamps may operate under very different dischargeconditions. The small size of subminiature fluorescent lamps may notallow for hot cathode operation, thus requiring efficient cold cathodeemitters. The buffer gas pressure in subminiature fluorescent lamps isoften on the order of 100 torr in order to limit electron loss to thelamp wall. By contrast, conventional fluorescent lamps typically utilizepressures on the order of 0.5-2.0 torr. Environmental concerns havenecessitated the investigation of lamp fill materials other thanmercury. In mercury-free fluorescent lamps, radiation is often producedby excimers of inert gases, such as neon, argon and xenon. In order toform excimers, a gas pressure of approximately 100 torr is required. Insubminiature fluorescent lamps utilizing cold cathodes, the operatinglife may be limited by sputtering. In addition, current limitations mayrestrict light output. These trends have produced a need for improvedcathode configurations.

The hollow cathode configurations disclosed in the prior art are notsuitable for use in subminiature fluorescent lamps because of theirrelatively large sizes and because of the relatively high pressuresutilized in subminiature fluorescent lamps.

Hollow cathodes have been studied in connection with other applications,such as excitation sources for gags lasers, ion sources, plasma jets,electron beams and plasma switches. In each case, a cathode with asingle, relatively large opening, or hollow, has been studied at low(subtorr) pressure.

SUMMARY OF THE INVENTION

According to the invention, a discharge device for operation in a gas ata prescribed pressure comprises a cathode and an anode spaced from thecathode, and electrical means for coupling electrical energy to thecathode and the anode. The cathode comprises a conductor having aplurality of micro hollows therein. Each of the micro hollows hascross-sectional dimensions selected to support a micro hollow dischargeat the prescribed pressure. Electrical energy is coupled to the cathodeand the anode at a voltage and current for producing micro hollowdischarges in each of the micro hollows in the cathode.

Each of the micro hollows preferably has a cross-sectional dimensionthat is on the order of the mean free path of electrons in the gas.Under these conditions, electrons undergo oscillatory motion within themicro hollows and produce substantially higher currents than a planarcathode. The micro hollow discharges operate independently of eachother.

The prescribed pressure for operation of the discharge device ispreferably in a range of about 0.1 torr to atmospheric pressure. Thedischarge device may include a discharge chamber for maintaining theprescribed pressure. When the discharge device is operated at or nearatmospheric pressure in air, the discharge chamber may not be required.

The discharge device may include a dielectric layer between the cathodeand the anode. The dielectric layer is preferably disposed of a surfaceof the cathode and is provided with openings aligned with the microhollows. The dielectric layer is preferably utilized when the spacingbetween the cathode and the anode is greater than about the mean freepath of electrons in the gas. The dielectric layer ensures that a glowdischarge between the cathode and the anode terminates in the microhollows.

According to a first application of the discharge device, a fluorescentlamp comprises a sealed, light-transmissive tube containing a gas at aprescribed pressure, and first and second spaced-apart electrodesmounted within the tube. The first electrode comprises a conductorhaving a plurality of micro hollows therein. Each of the micro hollowshas dimensions selected to support a micro hollow discharge at theprescribed pressure. The fluorescent lamp further includes electricalmeans for coupling electrical energy to the first and second electrodesat a voltage and current for producing micro hollow discharges in eachof the micro hollows in the first electrode. The fluorescent lamppreferably includes a phosphor coating on the inside surface of thelight-transmissive tube. The phosphor coating emits radiation having aprescribed spectrum in response to radiation generated by the dischargebetween the first and second electrodes. Each of the micro hollowspreferably has a cross-sectional dimension that is on the order of themean free path of electrons in the gas.

For AC operation of the fluorescent lamp, the second electrodepreferably comprises a conductor having a plurality of micro hollowstherein. Each of the micro hollows in the second electrode hasdimensions selected to produce a micro hollow discharge at theprescribed pressure.

The fluorescent lamp preferably includes a dielectric layer on thesurface of each electrode. Each dielectric layer has openings alignedwith the micro hollows.

In a second application of the discharge device, a radiation sourcecomprises a sealed discharge tube containing a gas at a prescribedpressure, first and, second spaced-apart electrodes mounted within thedischarge tube, add electrical means for coupling electrical energy tothe first and second electrodes. At least one of the electrodescomprises a conductor having a plurality of micro hollows. Each of themicro hollows has dimensions selected to produce a micro hollowdischarge at the prescribed pressure. In a preferred embodiment, theradiation source is an excimer lamp wherein the gas and the prescribedpressure are selected to emit radiation in a wavelength range of about80 to 200 nanometers.

In a third application of the discharge device, a laser for generatinglaser radiation at a predetermined wavelength comprises a first mirrorthat is substantially reflective at the predetermined wavelength, asecond mirror that is partially reflective and partially transmissive atthe predetermined wavelength, a chamber for enclosing a gas at aprescribed pressure between the first and second mirrors, and a laserpumping device positioned between the first and second mirrors. Thelaser pumping device comprises a cathode having at least one microhollow therein, the micro hollow having dimensions selected to produce amicro hollow discharge at the prescribed pressure, an anode spaced fromthe cathode and electrical means for coupling electrical energy to thecathode and the anode at a voltage and current for producing the microhollow discharge in the micro hollow. The laser pumping device providesan unobstructed optical path along the optical axis between the firstand second mirrors. The cathode may include a plurality of micro hollowsand the anode may include a plurality of openings aligned with the microhollows. In this case, each of the micro hollows defines an optical axisbetween the first and second mirrors for a generation of multiple laserbeams at the predetermined wavelength. Two or more of the laser pumpingdevices may be disposed along the optical axis between the first andsecond mirrors.

In a fourth application of the discharge device, a light sourcecomprises a sealed discharge chamber containing a gas at a prescribedpressure, a cathode mounted within the discharge chamber and an anodespaced from the cathode. The cathode comprises a conductor that definesan array of micro hollows. Each of the micro hollows has across-sectional dimension selected to support a micro hollow dischargeat the prescribed pressure and has an axial dimension that issubstantially less than the cross-sectional dimension. The light sourcefurther comprises electrical means for coupling electrical energy to thecathode and the anode at a voltage and current for producing microhollow discharges in each of the micro hollows in the cathode. The lightsource is preferably configured as a thin, flat light source.

The light source may be.configured as a flat fluorescent light source,including a phosphor coating on a light-transmissive portion of thedischarge chamber. The phosphor coating emits radiation having aprescribed spectrum in response to radiation generated within the microhollows.

In a preferred embodiment, the cathode of the flat light sourcecomprises a wire mesh including spaced-apart conductors which define themicro hollows. Alternatively, the cathode may comprise a conductivepattern formed on a light-transmissive substrate, the conductive patterncomprising a grid of spaced-apart conductive lines.

In an additional application, the discharge device of the presentinvention can be configured as an electron source for generatingmultiple electron beams. In a further application, the discharge deviceis configured as an ion source for generating multiple ion beams.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the accompanying drawings, which are incorporated herein by referenceand in which:

FIG. 1 is a schematic diagram of a discharge device in accordance withthe present invention;

FIG. 2 is a graph of current as a function of voltage for the dischargedevice, illustrating the high glow mode and the low glow mode;

FIG. 3 illustrates an experimental setup for evaluation of the dischargedevice of the present invention;

FIG. 4 is a graph of voltage as a function of current for a cathodehaving a single hole, and an anode-cathode separation of 2.5centimeters;

FIG. 5 is a graph of voltage as a function of current for a cathodehaving a single hole, and an anode-cathode separation of 5 centimeters;

FIG. 6 is a graph of voltage as a function of current for a cathodehaving four holes, and an anode-cathode separation of 2.5 centimeters;

FIG. 7 is a graph of voltage as a function of current for a cathodehaving four holes, and an anode-cathode separation of 5 centimeters;

FIG. 8 is a graph of voltage as a function of current for a cathodehaving eight holes, and an anode-cathode separation of 2.5 centimeters;

FIG. 9 is a graph of voltage as a function of current for a cathodehaving eight holes, and an anode-cathode separation of 5 centimeters;

FIG. 10A is a graph of voltage as a function of current for a cathodehaving three holes and an anode-cathode separation of 0.2 millimeter,for pressures in the range of 1.5 torr to 6 torr and for the low currentglow mode;

FIG. 10B is a graph of current as a function of pressure for the lowcurrent glow mode at 320 volts, for three hole and one hole discharges;

FIG. 11 is a graph of voltage as a function of current for a cathodehaving four holes at a pressure of two torr, showing a transition to thehigh current mode;

FIG. 12 is a simplified schematic diagram of a subminiature fluorescentlamp in accordance with the present invention;

FIG. 13 is an axial view of the cathode in the subminiature fluorescentlamp of FIG. 12;

FIG. 14 is a schematic, cross-sectional diagram of a gas laser using thedischarge device of the present invention for optical pumping;

FIG. 15 is an axial view of an array of micro hollows;

FIG. 16 is a partial cross-sectional view of a discharge device suitablefor use as an excimer light source in accordance with the presentinvention;

FIG. 17 is a partial cross-sectional view of a flat fluorescent lightsource in accordance with the present invention;

FIG. 18 is a partial illustration of the mesh cathode of FIG. 17; and

FIG. 19 is a partial cross-sectional view of an alternate embodiment ofthe flat fluorescent light source.

DETAILED DESCRIPTION

A discharge device in accordance with the present invention is shownschematically in FIG. 1. The discharge device includes a cathode 10 andan anode 12 mounted within a discharge chamber 14. The discharge chamber14 is typically sealed and contains a gas at a prescribed pressure, P.The pressure P is typically in a range of about 0.1 torr to atmosphericpressure. In some cases, the discharge chamber 14 may have an opening topermit gas flow or to permit passage of a charged particle beam, asdescribed below. In general, the discharge chamber 14 maintains thepressure P between anode 10 and cathode 12 within a desired range. Whenthe discharge device is operated at atmospheric pressure in air, thedischarge chamber may be omitted. A power source 18 connected to cathode10 and anode 12 supplies electrical energy to the discharge device.

The cathode 10 comprises an electrically conductive material having oneor more holes, referred to herein as micro hollows 20. Preferably,cathode 10 includes a plurality of micro hollows 20 for increasedcurrent capability. The micro hollows 20 are formed in a surface 22 ofcathode 10 which may be flat or curved and which faces anode 12. Each ofthe micro hollows 20 has a diameter, D, and extends from surface 22 intocathode 10. As described below, the diameter D of each of the microhollows 20 is selected to support a micro hollow discharge at theprescribed operating pressure within discharge chamber 14. The diameterD is defined as the diameter of a cross-section of the micro hollow in aplane parallel to surface 22 and perpendicular to a longitudinal axis 24of the micro hollow 20. In some cases, the cross section of the microhollow may not be circular. However, for ease of understanding,reference is made herein to the diameter D of the micro hollow. Wherethe cross section is not circular, it will be understood that thecross-sectional dimension is selected in the manner described below tosupport a micro hollow discharge. As shown in FIG. 1, the micro hollows20 may be closed at one end. However, the micro hollows can be open atboth ends within the scope of the present invention.

The shape of the micro hollows is not critical. The micro hollows may,for example, be formed by drilling, thus defining a generallycylindrical shape, at least initially. The micro hollows preferably havea circular cross section in a plane parallel to surface 22 of cathode10. Alternatively, the cross section of the micro hollow can be oval,square, rectangular or slit shaped. It has been reported that theinitial cylindrical shape of the micro hollow transforms itself into aspherical shape through sputtering and deposition. In cases where themicro hollow does not have a uniform diameter along the micro hollowaxis 24, the diameter D is defined at surface 22. The lifetime of themicro hollow cathode is expected to be long because of low cavityerosion, due to a balance of sputtering and redeposition inside themicro hollow.

The diameter D of each of the micro hollows 20 is selected to support amicro hollow discharge within each of the micro hollows 20. Morespecifically, the diameter D is selected such that the cathode fallregion extending from the inner wall of the micro hollow is on the orderof the hole radius. The cathode fall region is defined as a region ofincreased electric field near the cathode surface. The intensity anddistribution of the electric field is such that the ions acceleratedtoward the cathode gain sufficient energy to provide for emission ofsecondary electrons from the cathode, which are needed for aself-sustained glow discharge. The electrons emitted from the cathodesurface within the micro hollow are accelerated in the cathode fallregion toward the micro hollow axis 24. These electrons cross the axisand enter the cathode fall region on the opposite side of the axis,where they are reflected and accelerated across the axis again. Theoscillatory motion of the so called “pendel” electrons allows them tounload most of the energy gained in the cathode fall region throughexciting and ionizing collisions inside the micro hollow before driftingto the anode. The large ionization rate in a relatively small volumecauses a high plasma density on the discharge axis inside the microhollows 20 and consequently a high current. A “micro hollow discharge”occurs when electrons undergo oscillatory motion within the microhollows. As used herein, the term “micro hollow” refers to a cathodehole having a cross-sectional diameter D in a plane parallel to thecathode surface. The hole diameter D times pressure P in the dischargechamber must be in a range of 0.1 torr-centimeter to 10torr-centimeters, depending on the gas type, electrode material anddesired mode of operation (high or low glow mode). In the dischargedevice of FIG. 1, the current is found to be several orders of magnitudegreater than the current for a planar cathode, and the voltage is lower.

The conditions for a micro hollow discharge as described above are metwhen the hole diameter D is on the order of the mean free path of theelectrons in the gas. The mean free path depends on the type of gas andthe gas pressure in the discharge chamber 14, and is approximately equalto the dimension of the cathode fall region. Optimum micro hollowdischarge conditions are obtained when the diameter D of the microhollows is about twice the mean free path of electrons in the gas in thedischarge chamber. However, it will be understood that other values ofdiameter D can be used within the scope of the invention. Preferably,the diameter D is in a range of about 0.1 to 10 times the mean free pathof electrons in the gas, but the diameter D is not limited to thisrange.

The discharge parameters vary with the product of pressure P times holediameter D. The range of PAD for which the micro hollow discharge isstable for rare gases was found to be on the order of 0.1 to 10torr-centimeters.

It is believed that most of the micro hollow discharge current isgenerated in a region of the micro hollow wall that extends from thesurface 22 of cathode 10 to a depth that is about three times thediameter D of the micro hollow. Thus, little additional current isobtained when the depth, L, of the micro hollow is greater than aboutthree times the diameter D. However, a micro hollow discharge occurseven when the depth L of the micro hollow is smaller than the diameterD, with a reduction in discharge current.

The number of micro hollows 20 is selected to produce a desired totalcurrent at the operating voltage. It has been found that the microhollows 20 can be closely spaced without adversely affecting theindependent operation of the discharges.

Also shown in FIG. 1 is a dielectric layer 30 on surface 22 of cathode10. The dielectric layer 30 is required when the spacing, S, betweencathode 10 and anode 12 is greater than about the mean free path ofelectrons in the gas. When the spacing S exceeds this value and thedielectric layer 30 is not utilized, the glow discharge between cathode10 and anode 12 may terminate on surface 22 of cathode 10, rather thanin the micro hollows 20. Preferably, the dielectric layer 30 coverssurface 22 and surrounds micro hollows 20. The dielectric layer 30 can,for example, be a mica layer affixed to surface 22, or can be depositedon surface using known deposition techniques. When the spacing S is lessthan about the mean free path of electrons in the gas, the dielectric,layer 30 may be omitted.

The anode 12 can have any desired configuration which permits anelectric field to be established in the vicinity of cathode 10.Preferably, the anode 12 is planar and has an area that is approximatelyequal to the area of cathode 10, so that the spacing S between cathode10 and anode 12 is approximately uniform over the area of surface 22.The planar anode can optionally have holes aligned with the microhollows to provide a path for radiation generated by the discharge, agas flowing through the micro hollows, or an electron or ion beam.

In cases where the power source 18 supplies an AC voltage to thedischarge device, the anode 12 can be provided with micro hollows in thesame manner as cathode 10. To avoid confusion in the AC configuration,cathode 10 is called “electrode 10”, and anode 12 is called “electrode12”. Electrode 10 functions as a cathode during those half cycles of theAC voltage when electrode 12 is positive with respect to electrode 10,and electrode 12 functions as a cathode during those half cycles of theAC voltage when electrode 10 is positive with respect to electrode 12.By providing electrodes 10 and 12 with micro hollows as described above,micro hollow discharges are obtained on both half cycles of the ACvoltage.

The gas in the discharge chamber 14 may, for example, be an inert gassuch as argon, neon or xenon. However, any desired gas can be utilized,including mixtures of gases. As noted above, the pressure withindischarge chamber 14 is preferably in a range of about 0.1 torr toatmospheric pressure. A number of applications utilize pressures in arange of about 0.1 torr to 200 torr

The cathode 10 can be fabricated of any desired conductive material.However, materials with a high rate of secondary electron emissionthrough ion impact are preferred. Suitable materials of this typeinclude tungsten, barium oxide embedded in tungsten, thoriated tungsten,molybdenum and aluminum coated with oxygen. Materials, includingcomposite materials, characterized by a low electron work function aresuitable as cathode materials. Other suitable materials meeting theserequirements are known to those skilled in the art. In an alternativeconfiguration, the inside surfaces of micro hollows 20 are coated withmaterials that have high electron emissivity, and the remainder ofcathode 10 is fabricated of any desired conductive material.

The discharge chamber 14 can have any desired size and shape. Typically,the discharge chamber is sealed to maintain pressure P in the dischargeregion. The chamber 14 may be fabricated, at least in part, of amaterial that transmits radiation generated by the discharge. Thus, forexample, the discharge chamber 14 may be fabricated of alight-transmissive material, such as glass or quartz, or may have aradiation-transmissive window. In other embodiments, the, dischargechamber 14 may be configured such that gas at pressure P flows throughthe discharge region.

The power source 18 may supply a DC voltage, a pulsed voltage or an ACvoltage to the discharge device. For an AC voltage, a micro hollowdischarge occurs only on half cycles when the anode 12 is positive withrespect to the cathode 10, unless both electrodes have a micro hollowconfiguration as described above. The required voltage is typically in arange of about 300 to 600 volts. The micro hollow discharges have apositive voltage-current (V-I) characteristic over a large range ofcurrents and voltages, which permits operation of the micro hollows inparallel without ballast resistors. The micro hollow discharge has beenobserved to operate at currents up to 200 to 500 milliamps per microhollow.

The micro hollow discharges have been observed to have two glow modes,as illustrated in FIG. 2. In a low glow mode 36 at relatively lowvoltage and current levels, the plasma column is located on the axis ofthe micro hollow and appears as a slight glow in the micro hollow. In ahigh glow mode 38, the plasma column fills almost the entire microhollow and appears as a very bright discharge in the micro hollow. Thehigh glow mode occurs at higher current and voltage levels. Thedischarge switches abruptly from the low glow mode to the high glow modeas the voltage is increased. In both modes, the discharges are stableand do not influence each other. Spectral measurements of the high glowmode indicate the presence of spectral lines from the cathode material,thereby suggesting increased sputtering of the cathode material in thehigh glow mode. Besides the gas ions, the metal ions of the sputteredelectrode material contribute to the current flow and the secondaryelectron generation at the cathode.

The high and low glow modes refer to the discharges in, the microhollows 20. When the cathode 10 and the anode 12 have a spacing Sgreater than about the mean free path of electrons in the gas, a glowdischarge occurs in the region outside the micro hollows 20 betweencathode 10 and anode 12.

A set of experiments was performed to investigate micro hollow cathodedischarge in an argon-mercury environment with single and multiplecathode holes. A schematic diagram of the experimental configuration isshown in FIG. 3. A test chamber was defined by a glass tube 100 having alength of 23 cm and a diameter of 4 cm. The ends of glass tube 100 weresealed by stainless steel blocks 102 and 104. A cathode 110 and an anode112 located within the chamber could be varied in spacing between 0.1 cmand 15 cm. Molybdenum cathodes with 1, 4 and 8 holes of 0.7 mm diameterand 2.1 mm depth were used. A Cober Model 605P High Power PulseGenerator 116 was used to supply a 360 microsecond pulse at 30 Hz to thecathode 110. The voltage across the discharge-was measured using aTektronix P-6015 100CX High Voltage Probe, and the current across theload was measured using a Tektronix AM503 current probe.

Different gas pressures, different cold spot temperatures (mercurypressure), different electrode separations and different numbers ofmicro hollows were studied. FIG. 4 shows the voltage-current (V-I)characteristics of a cathode having a single micro hollow, with 2.5 cmelectrode separation, a pressure of 3 torr of a mercury-argon mixtureand cold spot temperatures of 15° C. and 25° C. A constant voltagedischarge was observed at the low current level, for example, less than240 milliamps for T=15° C. and less than 260 milliamps for T=25° C.Positive V-I characteristics were obtained at a higher current level. Ahigher cold spot temperature promotes a lower current level when thevoltage is kept constant. At larger electrode separation, thatdifference disappears, and the V-I characteristics overlap. FIG. 5 showsthe V-I characteristics of a cathode having a single micro hollow, with5.0 cm electrode separation, a pressure of 3 torr and cold spottemperatures of 15° C. and 25° C. The threshold current for positive V-Icharacteristics is higher for higher cold spot temperatures, as shown inFIG. 5.

FIG. 6 shows the V-I characteristics of a cathode having four microhollows, with 2.5 cm separation between electrodes, a pressure of 3 torrand cold spot temperatures of 15° C. and 25° C. FIG. 7 shows the V-Icharacteristics of a cathode having four micro hollows, with 5.0 cmseparation between electrodes, a pressure of 3 torr and cold spottemperatures of 15° C. and 25° C. A discharge with four micro hollowsdemonstrated unstable conditions at low current levels. In the currentrange below 300 milliamps at 2.5 cm electrode separation and below 350milliamps at 5.0 cm electrode separation, the four micro hollow cathodedischarge switched consecutively from the low glow mode into the highglow mode. After all of the discharges operated in the high glow mode,the V-I characteristic became positive and stable. All four microhollows were then operating in parallel with about equal lightintensity. The threshold current levels correspond to 375 volts,respectively. The current at 25° C. was higher than at 15° C. when thevoltage was maintained at a constant level.

FIG. 8 shows the V-I characteristics of a cathode having eight microhollows with 2.5 cm electrodes separation, a pressure of 3 torr and coldspot temperatures of 15° C. and 25° C. FIG. 9 shows the V-Icharacteristics of a cathode having eight micro hollows with 5.0 cmelectrodes separation, a pressure of 3 torr and cold spot temperaturesof 15° C. and 25° C. The cathode having eight micro hollows operated ina parallel and stable manner for currents higher than 400 milliamps,corresponding to a voltage of 375 volts, at 2.5 cm separation (FIG. 8)and for currents higher than 500 milliamps, corresponding to a voltageof 350 to 375 volts, at 5.0 cm separation (FIG. 9), after all eightmicro hollows transferred into the high glow mode. The current levelobtained with eight micro hollows is only slightly higher than with fourmicro hollows. For example, at 450 volts, four micro hollows operate at700 milliamps at 15° C. and 800 milliamps at 25° C., while eight microhollows operate at give 950 milliamps and 950 milliamps for temperaturesof 15° C. and 25° C., respectively.

Another set of experiments was performed with a cathode having threeholes to study parallel operation of micro hollow cathode dischargedevices in a situation where the anode-cathode distance was less thanthe micro hollow diameter. Cathode holes having a diameter of 0.7 mm anda depth of 2.1 mm were drilled in a molybdenum disk. A molybdenum foil12.7 micrometers thick with four 2 mm holes was used as the anode. Theanode and cathode were separated by a 0.2 mm thick mica spacer. Thevoltage and current were measured as described above in connection withFIGS. 4-9. FIG. 10A illustrates the I-V characteristics of the threehole hollow cathode discharge with pressures between 1.5 torr and 6torr. The discharge exhibited two modes of operation, the first being asubmilliamp unstable glow mode indicated by the points below 1.0milliamp in FIG. 10B, and the second beings a low current glow modeindicated by the points above 1.0 milliamp in FIG. 10B. The discharge inthe unstable glow mode was a slight glow that occupied only the centerof the hole, and the discharge in the low current glow mode occupiedabout half the hole. FIG. 10B compares the current levels at a givenvoltage of the three hole discharge with a one hole discharge. Over therange of pressures shown in FIG. 10B, the ratio of three hole current toone hole current is about three, indicating the multiplication propertyof the micro hollow cathode discharge.

In another set of experiments with four holes with the same dimensionsas above, the transition between the low current glow mode and the highcurrent glow mode was observed. The high current glow mode was a verybright discharge that filled most of the hole. The discharge startedwith a low glow in each of the four holes, and as the voltage applied tothe discharge was increased, the individual holes switched to the highglow mode. FIG. 11 shows that a low glow mode was obtained at 400 voltsacross the discharge. The discharge current increased linearly until 500volts. Then, one of the holes transferred into the high glow mode, andthe voltage decreased to 460 volts. The discharge continued with onehole in high glow mode until 580 volts was reached. At this point, asecond hole transferred into the high glow mode and the voltagedecreased to 500 volts. A third hole did not transfer into the high glowmode until the voltage reached 580 volts. At that point, the voltageacross the discharge dropped to 480 volts. The fourth hole transferredto high glow mode when the discharge voltage reached 540 volts. Thedischarge voltage at this point decreased to 500 volts.

Spectra of the discharges were recorded at 3 torr argon pressure. Afirst spectrum was taken with all of the holes in the low glow mode, anda second was taken with three of the holes in the high glow mode. Thedischarges contained molybdenum lines when the high glow mode waspresent.

An application of the discharge device of the present invention is shownin FIGS. 12 and 13. The discharge device is configured as a fluorescentlamp for generation of visible light. The fluorescent lamp includes afirst electrode 210 and a second electrode 212 sealed within alight-transmissive tube 214, which may be glass. The electrodes 210 and212 are spaced apart and are preferably located at or near opposite endsof light-transmissive tube 214. Electrical conductors 216 and 218 extendfrom the exterior of light-transmissive tube. 214 to electrodes 210 and212, respectively, and permit connection of electrodes 210 and 212 to asource of electrical energy (not shown). The light-transmissive tube 214defines a sealed chamber that is maintained at a desired pressure duringoperation. The conductors 216 and 218 extend through vacuumfeedthroughs, as known in the art. The light-transmissive tube 214contains a fill material for supporting a low pressure discharge betweenelectrodes 210 and 212. The fill material is typically an inert gas,such as neon, argon or xenon, and mercury vapor. Typically, the insidesurface of light-transmissive tube 214 is coated with a phosphormaterial that emits visible light in response to ultraviolet radiationgenerated by the discharge within the tube. A variety of phosphormaterials are well known to those skilled in the art.

In the embodiment of FIGS. 12 and 13, the electrode 210 comprises agenerally disk-shaped conductor. The electrode 210 preferably has a flatsurface 226 that faces electrode 212 and has sufficient thickness forformation of micro hollows. An array of micro hollows 230 is formed inthe surface 226 of electrode 210. Each of the micro hollows 230comprises a hole having a prescribed diameter that extends from surface226 into electrode 210. The diameter of each of the micro hollows 230depends on the type of gas and the operating pressure within thedischarge device. A dielectric layer 228 is disposed on surface 226 ofelectrode 210. The dielectric layer 228 surrounds but does not covermicro hollows 230.

In the embodiment of FIGS. 12 and 13, the micro hollows 230 are closedat one end. However, the micro hollows can extend entirely throughelectrode 210 within the scope of the present invention. The shape ofthe micro hollows is not critical. The micro hollows may, for example,be formed by drilling, thus defining a generally cylindrical shape. Theelectrode 210 can be fabricated of any conductive material, but ispreferably fabricated of a low work function material that has highelectron emissivity.

The diameter, D, of each of the micro hollows 230 is selected, dependingon the operating pressure, P, and the gas type within thelight-transmissive tube 214, to produce a micro hollow discharge withineach of the micro hollows 230. In particular, the diameter D of each ofthe micro hollows is preferably on the order of the mean free path ofelectrons in the light-transmissive tube 214. For rare gases, thiscondition is met when the product P·D is in a range of about 0.1 to 10,where the pressure P is specified in torr and the diameter D isspecified in centimeters. The operating pressure and the type of gas areusually established by other design considerations, thus setting anallowable range of diameters for the micro hollows. Fluorescent lampstypically contain argon and mercury vapor. Conventional fluorescentlamps typically operate at pressures of 0.5 to 2.0 torr, whereassubminiature fluorescent lamps may operate at pressures of 20 to 200torr. By way of example, for a subminiature fluorescent lamp having apressure of argon and mercury in the range of 20 to 200 torr, the microhollows 230 preferably have diameters in the range of 0.5 cm to lessthan 50 micrometers. The number of micro hollows 230 is selected toproduce a desired total discharge current. Preferably, the micro hollowsare relatively uniformly distributed over surface 26, and the surface 26between micro hollows is covered by dielectric layer 228.

In the fluorescent lamp, the radiation that stimulates the phosphorcoating on the light-transmissive tube 214 is generated in the positivecolumn between electrodes 210 and 212. The micro hollows function as asource of electrons, and generation of radiation within the microhollows is not important. For this reason, the micro hollow cathode ispreferably operated in the low glow mode for fluorescent lampapplications.

The electrode 210 is configured to function as a cathode for emission ofelectrons when it is biased negatively with respect to electrode 212.For typical fluorescent lamp applications, the electrode 212 isfabricated with an array of micro hollows and a dielectric layer in thesame manner as electrode 210. In this configuration, an AC voltage isapplied between conductors 216 and 218. Electrode 210 functions as acathode during those half cycles of the AC voltage when electrode 212 ispositive with respect to electrode 210, and electrode 212 functions as acathode during those half cycles of the AC voltage when electrode 210 ispositive with respect to electrode 212.

In another embodiment, the electrode 212 is not fabricated with an arrayof micro hollows and has a conventional anode configuration. In thisembodiment, electrode 212 functions continuously as an anode, andelectrode 210 functions continuously as a cathode. A DC voltage or apulse train is applied between conductors 216 and 218 in thisconfiguration.

The electrodes 210 and 212 are preferably fabricated of a material witha high rate of secondary emission through ion impact. Preferredmaterials include tungsten, barium oxide embedded in tungsten, thoriatedtungsten and molybdenum. Materials, including composite materials,characterized by a low electron work function are suitable as electrodematerials.

A variety of different gases can be utilized in the fluorescent lamp ofFIGS. 12 and 13. Preferred gases include mercury vapor mixed with aninert gas, such as argon or krypton, an inert gas, such as neon, withoutmercury vapor, an excimer of an inert gas, such as Xe₂, vapors of sulfuror selenium, and combinations thereof.

In an example, the fluorescent lamp shown in FIGS. 12 and 13 isconfigured as a subminiature fluorescent lamp. The light-transmissivetube 214 has an outside diameter of 7 mm, and the spacing betweenelectrodes 210 and 212 is about 100 mm. The tube 214 contains argon andmercury at a pressure of about 100 torr. Each of the electrodes 210 and212 has a diameter of about 5 mm and is provided with about 20 microhollows 230. The micro hollows have diameters of about 50 micrometers.The lamp is expected to operate in the low glow mode at about 300 voltsand a current of about 200 milliamps. In another example of thefluorescent lamp, the light transmissive tube 214 is 100 mm long and hasan outside diameter of 3 mm. The tube 214 contains argon and mercury ata pressure of about 50 torr. Each of the electrodes 210 and 212 has adiameter of about 1 mm and is provided with about 10 micro hollows 230.The micro hollows have diameters of about 50 micrometers. The lamp isexpected to operate in the low glow mode at about 400 volts and acurrent of about 5 milliamps per micro hollow. In general, the spacingbetween electrodes can vary between 10 cm and 100 cm, the pressure canvary between 1 and 200 torr, the micro hollow diameters can vary between10 and 1000 micrometers, and the number of micro hollows can vary from 5to 50 in order to achieve currents of 5 to 100 milliamps and voltages of20 to 500 volts. The range of selected parameters provides dischargeconditions with minimum electrode sputtering, maximum light output (10to 1000 lumens) and extended life (500 to 5000 hours). This range isdefined by subminiature fluorescent lamp conditions and applications.

The fluorescent lamp of FIGS. 12 and 13 has been described in connectionwith subminiature fluorescent lamps which have relatively smalldimensions and which operate at relatively high pressure. However, thecathode having an array of micro hollows is not limited to applicationin subminiature fluorescent lamps. The cathode having an array of microhollows can be used in any fluorescent lamp where the operating pressurepermits an array of suitably dimensioned micro hollows to obtain desiredoperating characteristics. The size and number of micro hollows isselected for a given operating pressure and current requirement.Furthermore, the phosphor coating on the light-transmissive tube can beomitted when the discharge within the tube produces a desired radiationspectrum. Different fill materials can be utilized within the scope ofthe present invention. Specifically, mercury free fluorescent lamps canmore easily be realized in the micro hollow cathode array system, sincethe expected electron energy distribution function is enriched with thehigh energy electrons and therefore promotes excitation of higher energylevels of typical gases considered for mercury replacement. Ionizationis also enhanced in this discharge arrangement. The cathode having anarray of micro hollows can optionally be heated to increase electronemission further.

Another application of the discharge device of the present invention isas an excimer lamp, which generates far ultraviolet radiation, typicallyin the wavelength range of 80-200 nanometers. The excimer lamp can beused for water purification, pasteurization, waste treatment and surfacetreatment of materials. The excimer lamp typically operates at arelatively high pressure, on the order of 100 torr or higher andcontains a gas, such as xenon or neon, that forms dimers at highpressures. Other suitable gases include all other noble gases andmixtures of noble gases with halogens. For operation at pressures on theorder of one atmosphere, the micro hollows have diameters on the orderof 10 to 100 micrometers. The excimer lamp can have any desiredconfiguration such as, for example, the discharge device shown in FIG.1. Alternatively, the excimer lamp may have a configuration similar tothe fluorescent lamp shown in FIG. 12 or the flat light sources shown inFIGS. 17 and 19. Generally, since the part of the discharge outside themicro hollows does not contribute to the excimer radiation, it can beeliminated, thus forming a flat light source with the anode-cathodedistance shorter than the electron mean free path. All or a portion ofthe discharge chamber is fabricated of a material, such as quartz, thattransmits ultraviolet radiation at the wavelength generated within thedischarge chamber. This far ultraviolet radiation can be convertedinside the lamp into visible radiation by a specially designed phosphor.Although the efficiency of such a lamp is at present lower than theefficiency of standard fluorescent lamps, this environmentally friendlylamp fill makes it an attractive alternative.

The excimer lamp can also be implemented as a micro hollow dischargearray, as shown in FIG. 16. A conductive cathode 400 is provided with anarray of micro hollows 402, 404, 406, etc., as described above inconnection with FIG. 1. An anode 410 is spaced from cathode 400 by adielectric layer 412. A second dielectric layer 414 is formed on theopposite surface of anode 410. The anode 410 may be a thin metal film.The anode 410 and the dielectric layers 412 and 414 may, for example, beformed by sputter deposition on cathode 400. The anode 410 and thedielectric layers 412 and 414 have openings aligned with each of themicro hollows 402, 404, 406, etc.

A further application of the discharge device of the present inventionis as a miniature gas laser. As discussed above, the increased currentof hollow cathode discharges compared to glow discharges between planarparallel electrodes is believed to be due to the high ionization rate ofnonthermal electrons, which oscillate between opposite cathode surfacesinside the cathode hole. The high energy electrons may be used fortransverse pumping of miniature gas lasers, operated at gas pressures ofup to one atmosphere. These miniature gas lasers, which in size arealmost comparable to semiconductor lasers, may emit over a wide spectralrange which reaches into the ultraviolet. The hollow cathode dischargepumped lasers are expected to be efficient. The lifetime is expected tobe long because of low cavity erosion, due to a balance of sputteringand redeposition in the cathode hole.

The radially accelerated, nonthermal electrons in a cylindrical microhollow discharge unload most of their energy close to the axis of thecathode hole. This energy is close to the free fall energy of theelectrons, which corresponds to the value of the applied voltage. Forsubmillimeter micro hollow discharges, the forward voltage is about 100to 500 volts. An electron energy of tens up to several hundred electronvolts is optimum for collisional ionization and excitation of atoms andmolecules. Most of the cross sections for excitation peak at about thisvalue. When the micro hollow discharge is used for laser pumping, thehighest rate of excitation of the upper laser state is on the axis ofthe cathode hole, with a steep decay toward the wall of the microhollow. For micro hollow discharges where the initially cylindricalcathode hole may turn into a spherical one due to sputtering andredeposition of electrode material, the maximum energy deposition willoccur at the electron focal point rather than along a focal line. Inorder to avoid this nonhomogenous distribution, the cathode hole isshortened in length to a dimension that is significantly smaller thanits diameter. For a 100 micrometer diameter, a cathode hole length ofabout 25 micrometers is suitable. Transverse pumping with micro hollowcathode discharges provides a class of gas lasers which are almost ascompact as semiconductor lasers. An additional advantage of thesedevices is the low noise level of the laser intensity compared to thatof lasers pumped by conventional discharges. The noise may be reduced bytwo orders of magnitude.

The helium-neon laser is particularly appropriate for micro hollowdischarge pumping, because experimental results in capillary tubes withdiameters of approximately 1 mm show that optimum gain is obtained whenthe pressure times distance product is 0.36 torr-cm, a value close tothe optimum pressure times diameter product for micro hollow cathodeoperation. The optimum relative pressures of helium and neon depend onthe discharge diameter only. For a helium-neon laser pumped with 100micrometer diameter micro hollows, the optimum pressure is 36 torr, with32 torr of helium and 4 torr of neon. The optimum power of this laser isexpected to be about 0.5 microwatt for a 0.5 mm long, 100 micrometerdiameter micro hollow cathode pumped with continuous wave energy.

Micro hollow discharges are believed to be ideally suited as pumpsources for metal ion lasers, with metals such as cadmium, silver, gold,lead and others. Micro hollow discharges provide metal ions throughcontinuous sputtering, instead of thermal processes, to create asufficient metal vapor pressure. Lasing from ultraviolet to nearinfrared has been demonstrated with hollow cathode pumping in variousmetal ion lasers.

The micro hollow cathode discharge device of the invention may also beused for pumping of rare gas ion lasers. The pressure times distancevalue for rare gas ion lasers is close to the optimum pressure timesdiameter value for micro hollow discharges. A micro hollow cathodedischarge with micro hollows having diameters of 100 micrometers canpump a rare gas laser operated close to atmospheric pressure. Microhollow cathode discharges may also be used as pump sources for nitrogenlasers and rare gas halide excimer lasers.

A cross sectional view of a single micro hollow discharge pumpedminiature gas laser is shown in FIG. 14. Micro hollow discharge elements300, 302, and 304 are stacked along an optical axis 306 of the laser.Different numbers of micro hollow discharge elements can be utilized toprovide desired laser characteristics. The discharge elements 300, 302,and 304 are positioned between a totally reflecting mirror 310 and apartially reflecting mirror 312. The partially reflecting mirror 312permits transmission of a laser beam 314 from the laser. The reflectioncharacteristics of mirrors 310 and 312 are defined at the operatingwavelength of the laser.

The discharge element 300 includes a cathode 320 and an anode 322separated by a first dielectric layer 324. A second dielectric layer 326is formed on the opposite surface of anode 322. The cathode 320 isprovided with a micro hollow 330 having a diameter that is selected,based on the type of gas and gas pressure in the discharge region, tosupport a micro hollow discharge. For operation near atmosphericpressure, the diameter of micro hollow 330 is preferably on the order ofabout 10 micrometers. As noted above, the depth of the micro hollow 330is preferably less than its diameter to ensure relatively uniformpumping along optical axis 306. In a preferred embodiment, the cathode320 has a thickness on the order of 25 micrometers. The anode 322 andthe dielectric layers 324 and 326 have openings that are aligned withthe micro hollow 330 to provide an unobstructed path along axis 306. Theanode 322 and the dielectric layers 324, 326 can, for example, be formedby sputtering on cathode 320. Discharge elements 302 and 304 have thesame structure as discharge element 300. The discharge elements 300,302, and 304 are attached to each other with micro hollows 330 alignedto provide a laminated discharge structure. As noted above, more orfewer discharge elements can be utilized in the miniature gas laser ofFIG. 14.

An axial view of an array of micro hollows configured as an array ofminiature gas lasers is shown in FIG. 15. The laser array includes arrayelements 340, 342, 344, etc., each of which may be constructed as shownin FIG. 14 and described above. The laser array may have any desirednumber of elements and may have a regular pattern of rows and columns,or may have an irregular pattern. Each discharge element of the laserarray may be constructed using conventional microlithography techniques.The discharge elements can be bonded together to provide the laminatedstructure shown in FIG. 14. The laser array generates multiple laserbeams.

In yet another application of the discharge device of the presentinvention, the micro hollow cathode discharge array is used as anelectron source or an ion source. As described above, electrons and ionsare generated within the micro hollows of the micro hollow cathode. Withreference to FIG. 16, electrons generated within micro hollows 402, 404,406, etc. are accelerated in a direction indicated by arrow 420, andions are accelerated in a direction indicated by arrow 422.

In a further application of the discharge device of the presentinvention, the micro hollow array is utilized in a thin, flat lightsource, typically a fluorescent light source. In this application, amicro hollow cathode is made of a grid of conductors, such as a wiremesh, having spacings which are preferably in the submillimeter range.The cathode is in close proximity to a planar anode. The flat lightsource can, for example, be used for backlighting of a display. Themicro hollows are formed as rings rather than cylindrical holes. Themicro hollows are implemented in accordance with the invention asdescribed above, but have small axial dimensions that are substantiallyless than their cross-sectional dimensions. The micro hollows may be,but are not required to be, open at both ends. Uniformity of thedischarge distribution in the micro hollows depends on the gas type, gaspressure, applied voltage.and the mesh or grid size. While the lightsource is described as being flat, it will be understood that thecomponents can be curved in a desired contour, if desired.

Display systems which utilize liquid crystals require some form ofbacklighting. This is conventionally achieved by tubular fluorescentlamps with optical elements such as reflectors, collimators anddiffusers. The discharge device of the present invention is utilized byplacing an array of micro hollow discharges directly behind a phosphorcoating to achieve relatively homogeneous illumination. The micro hollowcathode may consist of a metal mesh with openings in the submillimeterrange placed between the phosphor coating and a planar metal anode.

Because of the positive voltage-current characteristics of micro hollowdischarges, it is possible to operate them in parallel. The microhollows do not necessarily have an extended depth in the cathodematerial but may have the form of a ring. Even the cylindrical shape ofthe micro hollows is not a precondition for micro hollow cathodedischarges. The micro hollow shape may be quadratic, rectangular orbeehive style. Thus, metal meshes with openings in the submillimeterrange can be utilized in.a micro hollow cathode array. The anode can bea planar conductor separated from the cathode by a distance which iscomparable to or smaller than the cross-sectional dimensions of themicro hollows.

A partial cross-sectional view of a flat light source in accordance withthe present invention is shown in FIG. 17. A discharge chamber 500includes a light-transmissive wall 502 and a conductive wall 504. In theembodiment of FIG. 17, the light transmissive wall 502 and theconductive wall 504 are planar, parallel, spaced-apart sheets and areclosely spaced. The light-transmissive wall 502 and the conductive wall504 are sealed around their edges to define a sealed discharge volume. Acathode is positioned in the discharge chamber betweenlight-transmissive wall 502 and conductive wall 504. A phosphor coating506 may be applied to the inside surface of light-transmissive wall 502.A gas at a prescribed pressure is sealed within the discharge chamber500.

In the embodiment of FIG. 17, the cathode is an electrically-conductivemesh 508. The mesh 508 comprises a grid of spaced-apart wires or otherconductive strips which define a plurality of micro hollows. Morespecifically, with reference to FIG. 18, wires 510, 512, 514 and 516define a micro hollow 520. The wires 510 and 512 are parallel to eachother and are perpendicular to wires 514 and 516. The micro hollow 520in the example of FIG. 18 has a square cross-sectional shape with sidesequal to the spacing between the mesh wires. The axial depth of microhollow 520 is defined by the diameters of mesh wires 510, 512, 514 and516. The wires of the mesh 508 similarly define an array of microhollows, such as micro hollows 522, 524, 526, etc. The spacing betweenadjacent micro hollows is determined by the mesh wire diameter.

The mesh 508 is spaced from light-transmissive wall 502 by a dielectricspacer 530 and is spaced from conductive wall 504 by a dielectric spacer532. It will be understood that dielectric spacers 530 and 532 may belocated as required to maintain a desired spacing of mesh 508 withrespect to light-transmissive wall 502 and conductive wall 504. Thedielectric spacers 530 and 532 may, for example, be in the form ofelongated strips.

In operation, a voltage is applied between mesh 508, which functions asthe cathode of the discharge device, and conductive wall 504, whichfunctions as the anode. A micro hollow discharge is produced in each ofthe micro hollows 520, 522, 524, 526, etc. defined by the mesh 508. Theradiation generated by the micro hollow discharges stimulates emissionof visible light by phosphor coating 506. The light emitted by thephosphor coating 506 passes through light-transmissive wall 502 andappears as a generally uniform planar light source.

In the light source of FIG. 17, the fill gas may be a noble gas withmercury vapor, with dominant emission in the ultraviolet. Other suitablegases include inert gases, such as xenon, krypton and argon, or theirexcimers which would emit ultraviolet radiation, visible radiation or acombination of visible and ultraviolet radiation. The micro hollowcathode discharge enhances the high energy tail in the electron energydistribution function, allowing for more efficient excitation of excimerstates than conventional discharges. Molecular gases, such as nitrogen,oxygen or air, and sulfur or selenium vapors, and their mixtures withinert gases, may be used in the flat light source. The gas pressuredepends on the diameter of the cathode holes. For a mesh with 200micrometer openings and 50 micrometer wire diameter, the pressure ispreferably in a range of about 10 to 500 torr. The applied voltage is onthe order of 400 volts DC or pulsed. Preferably, the mesh spacing is ina range of 10-500 micrometers, which depends on the gas and gaspressure. In cases where it is not necessary to change the wavelength ofthe radiation generated in the micro hollows, the phosphor coating 506may be omitted.

Experiments were performed to study the gas discharge between a planarelectrode and a mesh electrode for utilization as a flat light source.The experimental setup included a vacuum chamber which included a planaranode made of tungsten impregnated with barium, and a nickel mesh withquadratic openings of 0.206 mm width separated by 0.044 mm wide metalbars, or strips, of 0.0014 mm thickness. The spacing between theelectrodes was on the order of 0.15 mm, determined by a mica spacerhaving an opening of about 2.5 mm. The gas was air at a pressure of 37.5torr. A voltage pulse with a droop of about 10% over the entire durationof 0.4 milliseconds was applied to the electrodes, and the currentthrough the discharge was monitored with a current viewing resistor.Simultaneously, the discharge was observed by a CCD camera connected toa magnifying system.

The results were as follows. At an applied voltage of 384 volts and withthe mesh biased negatively, the current at the beginning of the pulsewas measured at 33 milliamps. The current decayed to about half thisvalue over the duration of the voltage pulse, indicating a nonlineardependance of the current on the voltage. Discharges developed in themesh openings. Two types of discharges were observed: a dim dischargecentered in the mesh openings in a majority of the holes, and a bright,centered discharge in a small number of holes. With an increasing numberof pulses, the dim discharges became brighter, and the initially brightdischarges lost intensity. The current did not change significantlyduring the transition phase from inhomogeneous to more homogeneous lightdistribution. Continuous operation in this mode leads to substantialerosion of the mesh. In another experiment after more than 500,000pules, the 0.0014 mm bar was completely eroded at the position of thebrightest discharge. These results indicate that the flat light sourceneeds to be operated in a low glow mode to avoid erosion. This alsoguarantees long lifetime and preferable operation for lightingapplications. Experiments with reverse polarity (the planar electrode504 functioning as the cathode) showed a homogeneous glow at a lowercurrent of 16 milliamps and reduced intensity at the same voltage andpressure indicated above.

An alternate embodiment of the flat light source is shown in FIG. 19.Like elements in FIGS. 17 and 19 have the same reference numerals. Inthe embodiment of FIG. 19, a cathode 550 is formed as a conductivepattern on a transparent substrate 552. The cathode 550 includes a gridof spaced-apart conductive lines 556, 558, 560, etc. which define microhollows 564, 566, etc. The conductive pattern of cathode 550 can haveany desired configuration for defining a plurality of micro hollows. Theconductive pattern may formed using conventional microlithographytechniques. In the embodiment of FIG. 19, the substrate 552 functions asa support for the cathode 550. In other respects, the discharge deviceof FIG. 19 is similar to the discharge device shown in FIG. 17 anddescribed above.

The flat light sources of FIGS. 17-19 may have a, thickness on the orderof one millimeter. As noted above, the light sources shown in FIGS.17-19 may be flat or may have a desired curvature.

Generally, the devices of the present invention can be operated in thelow glow mode and the high glow mode as described above. However, onlythe low glow mode promises long lifetimes and operation determined bythe fill gas. In the high glow mode, the lifetime is limited, and theelectrode vapor determines the characteristic of the discharge. This maybe desirable when metal vapor radiation is required.

While there have been shown and described what are at present consideredthe preferred embodiments of the present invention, it will be obviousto those skilled in the art that various changes and modifications maybe made therein without departing from the scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A cathode for use in a discharge device thatoperates in a gas at a prescribed pressure, comprising a conductiveelement having a surface and at least one micro hollow formed in saidsurface, said at least one micro hollow having a diameter that is on theorder of the mean free path of electrons in said gas, said cathodefurther comprising a dielectric layer on said surface, said dielectriclayer having an opening aligned with said at least one micro hollow. 2.A cathode for use in a discharge device that operates in a gas at aprescribed pressure, comprising a conductive element having a surfaceand a plurality of micro hollows formed in said surface, each of saidmicro hollows having a cross-sectional dimension that is on the order ofthe mean free path of electrons in said gas.
 3. A cathode as defined inclaim 2 further comprising a dielectric layer on said surface of saidconductive element, said dielectric layer having opening aligned withsaid micro hollows.
 4. A cathode as defined in claim 2 wherein each ofsaid micro hollows has a depth of at least three times thecross-sectional dimension of each of said micro hollows.
 5. A cathode asdefined in claim 2 wherein each of said micro hollows has a generallycylindrical initial shape and is open at one end.
 6. A cathode asdefined in claim 2 wherein each of said micro hollows has a generallycylindrical initial shape and is open at both ends.